Liquid flow meter

ABSTRACT

A liquid flow meter for directly measuring the velocity of a liquid is disclosed. The liquid flow meter includes a pair transducers arranged facing each other in a conduit through which the liquid flows. The liquid flow meter also includes a transmitter means for causing the transducers to simultaneously transmit an acoustic wave packet directed for reception at the other transducer. A differential receiver means is also included whereby the differential receiver means has inputs each coupled to a corresponding one of the transducers for detecting an acoustic signal received thereby and determining a difference between the two received signals. The difference being related to the velocity of the liquid within the conduit. The transmitter means and the differential receiver means are each matched to the transducers to ensure substantial reciprocity to thereby substantially avoid phase and/or amplitude variations in the received signal. A method for measuring the velocity of a liquid is also disclosed. The method involves transmitting an acoustic wave packet between a pair of transducers that face each other, detecting an acoustic signal received by a pair of amplifiers and determining the time difference between the two received signals where the receiver amplifiers are matched to the transducers through resistive components.

FIELD OF THE PRESENT INVENTION

The present invention relates broadly to an electronic liquid flow meterand, in particular to a liquid flow meter for domestic and commercialuse.

BACKGROUND TO THE INVENTION

Traditional liquid flow meters in domestic and commercial use generallyinclude some mechanical arrangement such as a bellows, or a vane orimpeller which actuates a totalising mechanism. Generally, a mechanicaldial arrangement indicates the total volume of liquid that has passedthrough a meter Such mechanical arrangements are not highly accurate,especially at low flow rates. Accordingly, inaccuracies in low flow ratemeasurement can represent a substantial loss of income to the supplierof water or other liquids such as hydrocarbons.

Over recent years, there have been a number of proposals that utiliseelectronics technology so as to provide for substantial higher accuracyof the fluid flow measurement. Such systems generally incorporateultrasonic transducers that transmit ultrasonic signals both upstreamand downstream to measure the times of flight of the signals from whichthe relative speed of the fluid can be calculated. A further methodmeasures the phase change between two signals which are simultaneouslytransmitted from the transducers in order to calculate the speed of thefluid.

However, problems arise with the use of ultrasonic signals due tosubstantial variations in amplitude and phase of the waves transmittedand received by the ultrasonic transducer, resulting in inaccuratemeasurements. These variations can arise due to changes in temperature,a build-up of material on the transducer heads which affects impedancematching of circuit components, and also ageing and micro-cracking ofthe transduction elements.

Variations must be allowed for during the design and calibration oftransducer circuits with the “reciprocity theorem” being applied to thecircuit components. The Chambers Dictionary of Science & Technology(1991) defines the “reciprocity theorem” as “the interchange ofelectronic force at any one point in a network and the current producedat any other point results in the same current for the sameelectromotive force”. In application to acoustics, the theoremessentially says that a transmitter and a receiver may be swapped togive a reciprocal electro-acoustic transformation Existing arrangementsfail to achieve true reciprocity in ultrasonic transducer calibrations.Thus, accurate measurement, particularly at very low flow rates has beenunable to be achieved.

It is an object of the present invention to substantially overcome, orameliorate, one or more of the deficiencies of the above mentionedarrangement by provision of a liquid flow meter that is accurate over awide range of temperatures and operating conditions.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is disclosed aliquid flow meter for directly measuring the velocity of a liquid, saidliquid flow meter including:

a pair of transducers arranged facing each other in a conduit throughwhich the liquid flows;

transmitter means for causing the transducers to simultaneously transmitan acoustic wave packet directed for reception at the other saidtransducer;

differential receiver means having inputs each coupled to acorresponding one of said transducers for detecting an acoustic signalreceived thereby and determining a difference between the two receivedsignals, said difference being related to the velocity of liquid withinthe conduit, wherein the transmitter means and said differentialreceiver means are each matched to said transducers to ensuresubstantial reciprocity to thereby substantially avoid phase and/oramplitude variations in said received signals.

The acoustic wave packet as transmitted preferably comprises apredetermined plurality of cycles.

Preferably the differential receiver means comprises a pair of receiveramplifiers each coupled to a corresponding one of the transducers andoutputting the respective inputs of a differential detector. In aspecific embodiment the differential detector is formed by a transformerhaving the terminals of a primary winding coupled to the respectiveoutputs of the receiver amplifiers.

The differential detector preferably outputs a difference waveform,wherein the difference waveform is related to the velocity of the liquidwithin the conduit.

The liquid flow meter preferably includes a processing means, whereinthe processing means removes noise from the difference waveform andcalculates the difference between the two received signals.

The processing means farther preferably produces a sinusoidal pulsetrain at a predetermined frequency which is used to electrically excitethe liquid flow meter. Preferably the predetermined frequency is about 1MHz.

According to another aspect of the present invention there is provided aA method for directly measuring the velocity of a liquid, said methodcomprising the steps of:

simultaneously transmitting an acoustic wave packet between a pair oftransducers arranged facing each other in a conduit through which saidliquid flows;

a detecting an acoustic signal received by differential receiver meanshaving inputs each coupled to a corresponding one of said transducers;and

determining a difference between the two received signals, saiddifference being related to the velocity of liquid within the conduit,wherein the transmitter means and said differential receiver means areeach matched to said transducers to ensure substantial reciprocity tothereby substantially avoid phase and/or amplitude variations in saidreceived signals.

BRIEF DESCRIPTION OF DRAWINGS

A number of embodiments of the present invention will now be describedwith reference to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of the liquid flow meter of thepreferred embodiment;

FIG. 2 is a schematic block diagram of the liquid flow meter electronicscircuit of FIG. 1;

FIG. 3 is a drawing showing the two received transducer waveforms andthe resulting differential waveform;

FIG. 4 is a schematic circuit diagram of one implementation of theelectronics circuit of FIG. 2; and

FIG. 5 is a detailed circuit diagram of the electronics circuit of FIG.4.

DETAILED DESCRIPTION OF THE DRAWINGS

The preferred embodiment is a liquid flow meter that directly measuresthe velocity of a liquid by determining the time difference in receptionof two separately but simultaneously transmitted bursts of ultrasound inopposite directions in the same tube. Ultrasonic transducers arearranged within respective aerodynamic housings at each end of the tubeand can function as either ultrasound emitters or detectors. In thepreferred embodiment, the transducers are spaced apart by a distance of200 mm.

A suitable type of transducer that can be used with the preferredembodiment is a PZT ceramic transducer (manufactured by KB-Aerotech).The electronic liquid flow meter is provided with an electronics circuitwhich generates, detects and calculates a time difference between thetwo ultrasonic transducer waves,

FIG. 1 shows an overall block diagram of the liquid flow meter 1 of afirst embodiment which includes two transducers 10 and 20 which areconnected to an electronic circuit 30 which drives the transducers 10and 20 simultaneously and outputs signals 3, 5 and 7 having amplitudesor phases related to the velocity of the liquid. The output of theelectronics circuit 30 is fed into an analogue to digital converter(ADC) 40, for converting the signals 3, 5 and 7 into digital form forprocessing by a digital signal processor (DSP) 50. The DSP 50 performs alinear least squares fit on the signals 3, 5 and 7 to remove noisetherefrom to enable the actual flow rate to be calculated and displayed(not shown but known in the art). The DSP 50 outputs a digitalsinusoidal pulse train signal to a digital to analogue converter (DAC)60 which feeds back into the electronic circuit 30 in order to drive theelectronic circuit 30.

FIG. 2 shows an overall block diagram of the liquid flow meterelectronic circuit 30 of a first embodiment which includes an inputamplifier stage 70 connected to a matched resistor stage 80. The twotransducers 10 and 20 are also connected to the matched resistor stage80. A transducer receiving amplifier stage 90 is connected to the twotransducers 10 and 20 and feeds into a differential output amplifierstage 100. The transducer receiving amplifier stage 90 is alsoseparately connected to two output amplifier stages 110 and 120.

A sinusoidal pulse train at a preferred frequency of 1 MHz is applied tothe burst input 2 of the input amplifier stage 70 for a predeterminedperiod, typically 20 cycles. This preferred number of cycles is longenough to allow the transducer signals to settle at a constant amplitudeand yet, short enough so that reverberation between the transducers doesnot occur. In the preferred embodiment this sinusoidal signal issupplied from the DSP 50 through the DAC 60. The signal is buffered bythe input amplifier stage 70 and output simultaneously, via a matchedresistor stage 80, to the two ultrasonic transducers 10 and 20. Thetransducers 10,20, chosen for their stable characteristics, sendultrasonic signals in opposing directions, up and down a moving columnof fluid. A time delay will be endured by the signal travelling upstreamin comparison to that travelling downstream. Each transducer 10 and 20is configured to receive the ultrasonic waveform sent by the opposingtransducer and convert the respective received waveform into anequivalent electrical waveform. This signal is then fed into andbuffered by the transducer receiving amplifier stage 90. The outputs ofthe transducer receiving amplifier stage 90 feed into the differentialoutput amplifier stage 100, which measures the difference of the twoapplied waveforms to create a difference signal. The differential outputamplifier stage 100 then amplifies the difference signal and buffers tothe output. The two further amplifier stages 110 and 120 buffer copiesof each of the received signals of the transducers and output thesignals so that the raw signal amplitudes can be measured.

FIG. 3 shows a graph of the transducers received signals. The signaltransmitted by transducer 10 is illustrated in FIG. 3 as the downstreamsignal 3. A time delay can be observed in the upstream signal 5 whichwas transmitted by transducer 20. The differencing operation ofdifferential output amplifier stage 100 generates a signal 7 which isillustrated in the bottom graph of FIG. 3. The signal 7 has an amplitudethat is simply related to the time difference in reception of thesimultaneously transmitted downstream signal 3 and upstream signal 5.

FIG. 4 shows a circuit diagram of the liquid flow meter electroniccircuit 30 of the first embodiment. The input amplifier stage 70includes an operational amplifier (op amp) IC1 configured in anon-inverting amplifier configuration using negative feedback resistorR3, via lines 21 and 22, and a bootstrapped input, via componentsR2,C28. Op amp IC1 is preferably chosen and configured to have a highinput impedance and a very low output impedance which is in the range ofmilliohms. The high input impedance effectively decouples the signalinput from the circuit. This is supplemented by the bootstrap connectionmentioned above. R2 and C28 have a sufficient time constant such thatthe voltage at the negative input is equal to the voltage at thepositive input, and therefore the current through R2 and C28 isnominally equal to zero. The capacitor C28 providing further blocking atlower input frequencies.

The output of the input amplifier stage 70 is simultaneously applied totwo matched resistors R_(A) and R_(B), to the ultrasonic transducers 10and 20. The matching of the two resistors is critical so thatreciprocity holds.

The two transducers 10 and 20 are connected to two separate receivingamplifiers IC4 (via line 23) and IC5 (via line 24), respectively, whichembody the transducer receiving amplifier stage 90.

In the case of transducer 10, upon receiving the transmitted signal fromtransducer 20, the received signal is converted from an ultrasonic to anelectrical signal and applied to op amp IC4. Op amp IC4 is configured asa unity gain voltage follower. The op amp IC4 being chosen to have ahigh input impedance, a stable response and low noise. The capacitanceC14 on the output of IC4 is chosen large enough to act as a shortcircuit at the preferred frequency. The value of resistance R12 isnominal and does not affect the signal. This circuit is mirrored in thecase of transducer 20 for op amp IC5. Both of these amplifiersdifferentially drive a floating winding W1 of a transformer T1 (vialines 25 and 26), with a secondary winding W2 which provides an inputwith respect to ground to differential output amplifier stage 100, thelatter being formed using an op amp IC7, configured in a non-invertingmanner. A resistor R19 is connected across the secondary winding W2 ofthe transformer T1 and provides a current path for the transformer T1secondary induced e.m.f. current, thereby providing a voltage signal tothe positive input of op amp IC7, via line 27. The secondary winding W2of transformer T1, thereby registers the difference of the two appliedwaveforms which were input to either leg of the transformer T1 primary.Therefore, if both applied waveforms are in phase and of the sameamplitude, there will be no induced e.m.f. current in the secondarywinding W2 of transformer T1 and therefore no voltage input to the opamp IC7.

The difference signal, registered by the transformer T1 secondary isamplified by op amp IC7 and buffered for output through resistor R18.The transformer T1 and op amp IC7 being chosen to have a high commonmode rejection ratio so as to reduce any noise associated with thedifferencing operation of transformer T1.

The two amplifiers IC4 and IC5 which make-up the transducer receivingamplifier stage 90, individually feed into two further separate outputamplifier stages 110 and 120. The output of op amp IC4 feeds directly,via line 28, into the positive input of op amp IC6 which is configuredas a unity-gain voltage follower. The op amp IC6 buffers the transducer10 received signal and outputs a copy of the received signal, viaresistors R10 and R16. The arrangement of output amplifier stage 110,which includes op amp IC6, is mirrored for output amplifier stage 120with op amp IC5 feeding directly, via line 29, into op amp IC8. Op ampIC8 is also configured as a unity-gain voltage follower.

The differencing operation of the differential output amplifier stage100 generates an output signal 7 with an amplitude that is related tothe time difference upon reception between the two received transducersignals 3,5. This output signal 7 is measured by an analog-to-digitalconverter 40, in the preferred embodiment as seen in FIG. 1. In afurther embodiment, an oscilloscope may also be used to measure theoutput. The output is finally sent to a digital signal processor 50. Aleast-squares fit is carried out on the differential output signal tofurther reduce the noise in the estimate of its amplitude. This gives amore accurate estimate of the time difference between the two receivedsignals. Once the time difference has been calculated by the computer,the data can be inverted to give an estimate of the fluid flow rate.

Since a change in the amplitude of the received raw transducer signals3,5 will affect the amplitude of the difference signal as well, anychange in the raw signals needs to be counteracted. This is achieved inthe preferred embodiment by measuring any changes in the raw signalamplitudes and then dividing the differential output signal by thesecondary measured amplitude.

FIG. 5 shows a detailed circuit diagram of the liquid flow meterelectronic circuit 30 of the first embodiment, which shows all componentvalues and IC numbers. The positive and negative power supply terminalsof IC1 are tied to the +5 Vdc and the −5 Vdc regulated power supplyrails, respectively. Capacitors C1, C2, C5, C3 and C4 provide filteringand aid in regulating the voltage on the positive and negative supplyrails. A similar power supply and filtering arrangement is connected tothe other op amps IC4. IC5, IC6, IC7 and IC8. IC1 is an OPA621 chip(manufactured by Precision Monolithics) which has been chosen andconfigured to have a high input impedance and a very low outputimpedance. The values of R₂=100Ω and C₂₈=100 pf give a time constant of100 μs and therefore the circuit input current is nominally equal tozero.

The output impedance of IC1 is in the milliohm range and therefore thesignal seen by the matched resistors R_(A),R_(B) is nominally identicalto the input signal with the amplifier providing good current drive.

The matched resistor stage is formed of two 50 ohm matched resistorsR_(A), R_(B). The resistor R_(A) is configured as the parallelcombination of R₄=100 ohms, R₅=100 ohms and R₆. R6 is adjusted to matchthe two impedances. Typically a small outlying transistor (50T) orsurface mount resistor is used. This arrangement is mirrored in theparallel combination of R₇=100 ohms, R₈=100 ohms and R₉ for resistorR_(B). The resistors R₄, R₅, R₇ and R₈ being chosen to have tolerancevalues of ±0.1%. Critical matching can be carried out for both parallelcombinations through the adjustment of R₆ and R₉.

Op amp IC4 and IC5 are both AD829 chips (manufactured by Analog Devices)and were chosen to have a high input impedance and good phase andamplitude stability. The high input impedance further improves thematching of the impedance seen by the two transducers 10 and 20,minimises loading of the input driving stage and ensures reciprocity.The phase and amplitude stability ensure that the received signalsapplied to the transformer T1 primary winding W1 are identical to thoseseen by the transducers 10 and 20. Capacitor C15 is used for bandwidthcompensation. The capacitors C14 and C21 are both equal to 100 nf andwill act as a short circuit at the preferred operating frequency of 1MHz. The resistance R₁₂=100 ohms is a nominal value and will not affectthe signal. This value of R₁₂ is chosen in order to minimise noise andto provide good bandwidth response The power supply configuration,bandwidth compensation and output configuration of IC4 is substantiallymirrored for IC5.

Transformer T1 is a Philips 3C85 core transformer and has been chosenfor its high CMRR. The transformer has a turns ratio at 4:16 andtherefore provides amplification of the difference waveform which isapplied to its primary.

Op amp IC7 is an AD829 chip. Resistors R₁₁=2 k49 ohms and R₁₃=105 ohmsprovide a gain of approximately 25 for op amp IC7. Op amp IC7 amplifiesthe difference waveform and buffers it for output thereby decoupling theoutput stage. Compensation capacitor C30 sets the bandwidth response forop amp IC7 at a predetermined value.

Op amp IC6 and IC8 are also AD829 chips being chosen for their highinput impedance and low output impedance characteristic, decouple thecircuit from the output and provide good current drive. Compensationcapacitor C29 is used to adjust the bandwidth response of operationalamplifier IC6. The power supply configuration and bandwidth compensationof IC6 has essentially been mirrored for IC8.

FIG. 5 also shows two voltage regulator circuits which supply the +5 Vdcand −5 Vdc power supply rails for the operational amplifier stages.Voltage regulator IC2 has a +12 Vdc input to provide a +5 Vdc output.Capacitors C6, C8, and C9, C7 provide input and output filtering,respectively, for the voltage regulator IC2 and regulate the outputvoltage at the desired +5 Vdc level. This circuit is mirrored in thecase of the −5 Vdc rail, where voltage regulator IC3 is provided withfiltering capacitors C10 to C12.

The above-described embodiment has several advantages which are outlinedas follows:

First, due to the very low output impedance of IC1, the very high inputimpedance of IC4 and IC5, and the critical matching of the two 50 ohmtransducer input resistors, true reciprocity is achieved in thepreferred embodiment. As a result, the measurements are less susceptibleto drift caused by changes in the temperature of the transducers.

Second, the two output amplifier stages 110 and 120 further reduceinaccuracies in the calculated difference signal, by providing thereceived raw signal amplitudes to be measured. These measurements arethen taken into account when calculating the final output differenceamplitude and counter-act any discrepancies in the transmittedtransducer signal.

The foregoing describes only one embodiment of the present invention,and modifications, obvious to those skilled in the art, can be madethereto without departing from the scope of the present invention.

What is claimed is:
 1. A liquid flow meter for directly measuring thevelocity of a liquid, said liquid flow meter including: a pair oftransducers arranged facing each other in a conduit through which saidliquid flows: transmitter means coupled to said transducers viaresistive components, said transmitter means being configured to causesaid transducers to simultaneously transmit an acoustic wave packetdirected for reception at the other said transducer; and a pair ofreceiver amplifiers having inputs each coupled to a corresponding one ofsaid transducers and to a corresponding one of said resistivecomponents, said receiver amplifiers being configured to detect anacoustic signal received by a corresponding one of said transducers andto determine a time difference between the two received signals, saidtime difference being related to the velocity of liquid within theconduit, wherein the transmitter means and said receiver amplifiers areeach matched to said transducers, utilising said resistive components,to ensure substantial reciprocity to thereby substantially avoid phaseand/or amplitude variations in said received signals.
 2. The liquid flowmeter according to claim 1, wherein said acoustic wave packet astransmitted preferably comprises a predetermined plurality of cycles. 3.The liquid flow meter according to claim 1, wherein said receiveramplifiers are configured to output the respective inputs of adifferential detector.
 4. The liquid flow meter according to claim 3,wherein said differential detector outputs a time difference waveform,wherein said time difference waveform is related to the velocity of theliquid within the conduit.
 5. The liquid flow meter according to claim4, further including a processing means, wherein said processing meansremoves noise from said difference waveform and calculates a timedifference between said two received signals.
 6. The liquid flow meteraccording to claim 5, said processing means further produces asinusoidal pulse train at a predetermined frequency which is used toelectrically excite the liquid flow meter.
 7. The liquid flow meteraccording to claim 6, wherein said predetermined frequency is 1 MHz. 8.The liquid flow meter according to any one of claims 3, wherein saiddifferential detector is formed by a transformer having terminals of aprimary winding coupled to respective outputs of said receiveramplifiers.
 9. The liquid flow meter according to claim 1, wherein saidpassive ciruits are impedance matched.
 10. The liquid flow meteraccording to claim 1, wherein said impedances are matched during bothemission of said transmit signals and reception of said acousticsignals.
 11. A method for measuring the velocity of a liquid, saidmethod comprising the steps of: simultaneously transmitting an acousticwave packet between a pair of transducers arranged facing each other ina conduit through which said liquid flows; detecting a acoustic signalreceived by each of a pair of receiver amplifiers having correspondingfirst and second input nodes, each said node being coupled to an outputof said transmit amplifier stage via one of a pair of passive circuit,and being further connected to a corresponding one of said; anddetermining a time difference between the received acoustic signals,said time difference being related to the velocity of said liquid withinsaid conduit, wherein an impedance formed at one said node by an inputimpedance of a corresponding receiver amplifier and the correspondingone of said passive circuits is matched to the impedance at the othersaid node formed by The other receiver amplifier and other said passivecircuit.
 12. The method according to claim 9, further comprising thestep of outputting a difference waveform, said difference waveform beingrelated to the velocity of the liquid within the conduit.
 13. The methodaccording to claim 12, including the further steps of: removing noisefrom said difference waveform; and calculating a time difference betweensaid two received signals, utilising said receiver amplifiers, whereinsaid receiver amplifiers are configured to output the respective inputsof a differential detector.
 14. The method according to claim 9, whereina sinusoidal pulse rain is produced at a predetermined frequency. 15.The method according to claim 11, wherein said passive circuits areimpedance matched.
 16. The method according to claim 11, wherein saidimpedances are matched during both emission of said transmit signals andreception of said acoustic signals.
 17. A circuit adapted for use with aliquid flow meter, said circuit comprising: a transmit amplifier stagehaving an output; first and second receive amplifier stages havingcorresponding first and second input nodes, each said node beingconnected to said output of said tat amplifier stage via one of a pairof passive circuits so as to cause a transmit signal emitted from thetransmit amplifier stage to be passed simultaneously each said node; anda pair of transducers each coupled to one of said nodes, the pairs beingconfigured to convey acoustic signals therebetween upon exitation bysaid transmit signal, wherein an impedance formed at one said node by aninput impedance of said corresponding receive amplifier stage and thecorresponding one of said passive circuits is matched to the impedanceat the other said node formed by the other receive amplifier stage andother said passive circuit.
 18. A circuit according to clam 17, whereinsaid passive circuits are impedance matched.
 19. A circuit according toclaim 17, wherein said impedances are matched during both emission ofsaid transmit signals and reception of said acoustic signals.